Connect cities

#include <iostream>
#include <cstring>
#include <vector>

using namespace std;

const int N = 1e5 + 5;

struct dsu {
  int lab[N];

  dsu() {
    memset(lab, -1, sizeof(lab));
  }

  int find(int u) {
    return lab[u] < 0 ? u : lab[u] = find(lab[u]);
  }

  bool unite(int u, int v) {
    if ((u = find(u)) == (v = find(v))) {
      return false;
    }
    if (lab[u] > lab[v]) {
      swap(u, v);
    }
    lab[u] += lab[v];
    lab[v] = u;
    return true;
  }
};

int n, m;
dsu g;

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cin >> n >> m;
  for (int i = 1; i <= m; i++) {
    int a, b;
    cin >> a >> b;
    g.unite(a, b);
  }
  vector<pair<int, int>> ans;
  for (int i = 2; i <= n; i++) {
    if (g.unite(i, i - 1)) {
      ans.emplace_back(i, i - 1);
    }
  }
  cout << ans.size() << '\n';
  for (auto x : ans) {
    cout << x.first << ' ' << x.second << '\n';
  }
  return 0;
}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED

Test 4

Verdict: ACCEPTED

Test 5

Verdict: ACCEPTED

Test 6

Verdict: ACCEPTED

Test 7

Verdict: ACCEPTED

Test 8

Verdict: ACCEPTED

Test 9

Verdict: ACCEPTED

Test 10

Verdict: ACCEPTED

Test 11

Verdict: ACCEPTED

Test 12

Verdict: ACCEPTED